Convert grams of bromine to moles: Need formula weight. Use the chemical equation to relate moles of bromine to moles of aluminum

Transcription

1 99 * Given that we have 25.0 g of liquid bromine, how many grams of aluminum would we need to react away all of the bromine? Convert grams of bromine to moles: Need formula weight Use the chemical equation to relate moles of bromine to moles of aluminum Convert moles aluminum to mass: Need formula weight

2 100 You can combine all three steps on one line if you like! Things we can do: If we have and we need... Use... MASS SOLUTION VOLUME MOLES MOLES FORMULA WEIGHT MOLAR CONCETRATION (MOLARITY) MOLES OF A MOLES OF B BALANCED CHEMICAL EQUATION

3 101 Example: How many milliliters of 6.00M hydrochloric acid is needed to completely react with 25.0 g of sodium carbonate? 1 - Convert 25.0 g sodium carbonate to moles. Use FORMULA WEIGHT. 2 - Convert moles sodium carbonate to moles HCl. Use CHEMICAL EQUATION. 3 - Convert moles HCl to volume. Use MOLARITY.

4 102 Example: How many milliliters of 6.00M hydrochloric acid is needed to completely react with 25.0 g of sodium carbonate? 1 - Convert 25.0 g sodium carbonate to moles. Use FORMULA WEIGHT. 2 - Convert moles sodium carbonate to moles HCl. Use CHEMICAL EQUATION. 3 - Convert moles HCl to volume. Use MOLARITY. We've calculated the solution volume ( L), but the problem asks us for the answer in milliliters. We'll do a quick unit conversion.

5 103 propylene acrylonitrile Calculate how many grams of acrylonitrile could be obtained from 651 g of propylene, assuming there is excess NO present. 1 - Convert 651 grams propylene to moles. Use FORMULA WEIGHT. 2 - Convert moles propylene to moles acrylonitrile. Use CHEMICAL EQUATION. 3 - Convert moles acrylonitrile to mass. Use FORMULA WEIGHT.

6 104 How many ml of 0.250M potassium permangenate are needed to react with 3.36 g of iron(ii) sulfate? 1 - Convert 3.36 g iron(ii) sulfate to moles. Use FORMULA WEIGHT. 2 - Convert moles iron(ii) sulfate to moles potassium permangenate. Use CHEMICAL EQUATION. 3 - Convert moles potassium permangenate to volume. Use MOLARITY. We need to express the answer in ml, so convert.

7 105 CONCEPT OF LIMITING REACTANT - When does a chemical reaction STOP? Magnesium strip Flame, oxygen from air Magnesium oxide powder - When does this reaction stop? When burned in open air, this reaction stops when all the MAGNESIUM STRIP is gone. We say that the magnesium is LIMITING. - This reaction is controlled by the amount of available magnesium - At the end of a chemical reaction, the LIMITING REACTANT will be completely consumed, but there may be amount of OTHER reactants remaining. We do chemical calculations in part to minimize these "leftovers". These are often called "excess" reactants, or reactants present "in excess"

8 106 - To find the limiting reactant, calculate how much product would be produced from ALL given reactants. Whichever produces the SMALLEST amouht of product is the limiting reactant, and the smallest anount of product is the actual amount of product produced. Example: LIMITING REACTANT CALCULATIONS <- Formula weights If you start with 100. g of each reactant, how much calcium carbide would be produced? The reaction should produce 114 grams of calcium carbide. At that point, there is no more CaO left to react, and the reaction stops. There will still be some carbon left over, but it has nothing to react with. We say that CaO is LIMITING, and C is present IN EXCESS.

9 107 - Chemical reactions do not always go to completion! Things may happen that prevent the conversion of reactants to the desired/expected product! SIDE REACTIONS: This reaction occurs when there is a large amount of oxygen available... while this reaction is more favorable in low-oxygen environments!... so in a low-oxygen environment, you may produce less carbon dioxide than expected! TRANSFER AND OTHER LOSSES PERCENT YIELD - When isolating a product, losses may occur in the process. Example: filtering During each step of this process, some amount of product loss will occur! Pour through filter!... then scrape product off paper!

10 108 EQUILIBRIUM - Reactions may reach an equilbrium between prodcuts and reactants. We'll talk more about this in CHM 111. The net results is that the reaction will appear to stop before all reactants have been consumed! - All of these factors cause a chemical reaction to produce LESS product than calculated. For many reactions, this difference isn't significant. But for others, we need to report the PERCENT YIELD. Determined EXPERIMENTALLY! Calculated based on the limiting reactant. (The chemical calculations you've done up to now have been theoretical yields!)... the percent yield of a reaction can never be greater than 100% due to conservation of mass! If you determine that a percent yield is greater than 100%, then you've made a mistake somewhere - either in a calculation or in the experiment itself!

11 g 31.6 g ACTUAL <- Formula weights benzene nitric acid nitrobenzene 22.4 grams of benzene are reacted with excess nitric acid. If 31.6 grams of nitrobenzene are collected from the reaction, what is the percent yield? We know the actual yield (31.6 grams nitrobezene). We need to find the THEORETICAL yield. Calculate it by starting with the reactant grams benzene.